Observations suggestive of abnormal hemorheology in diabetic patients include the presence in some of "corpuscular" blood flow in retinal vessels and the occurrence of sludged flow with abnormal erythrocyte aggregation in conjunctival vessels. Similarities exist between disbetic retinopathy and retinopathies of other diseases with abnormal hemorheology. Preliminary in vitro observations and measurements on blood have shown abnormal aggregation of red blood cells in diabetic subjects. Such aggregates in vivo could obstruct precapillary arterioles to produce the multiple localized areas of retinal ischemia and/or hypoxia noted clinically and the presence of empty or obliterated retinal capillaries in diabetic subjects. Goals include study of: 1) Existence of abnormal hemorheology in diabetics, 2) changes in the microcirculation produced by altered blood rheology in diabetic retinopathy, 3) interaction of plasma components and formed elements in an attempt to identify those components responsible for altered blood rheology in diabetic retinopathy, and 4) possible therapeutic approaches to abnormal blood rheology to prevent the development of diabetic microangiopathy. In subjects with diabetic retinopathy, and in appropriate controls, clinical observation of conjunctival and retinal circulation will be conducted to evaluate rheologic behavior in vivo. High speed movies will be taken to study rheologic behavior in conjunctival vessels. Fluorescein angiography and detailed scotometry will be done to detect retinal vascular occulusions and flow patterns. Rheological studies will include rheoscopic observations and measurements of the red cell aggregation and viscous behavior of blood with quantification of strain rates required to disperse aggregates. These observations will be correlated with clinical, hematalogic and metabolic information. All of the data will be subjected to statistical analysis.